1. Field of the Invention
This invention pertains to active magneto bearings with velocity search coils in place of displacement (position) sensors, and the associated velocity feedback control scheme, particularly for radial types of active magnetic bearings.
2. Description of the Prior Art
In the prior art, displacement sensors have been used in the control systems for active magnetic bearings. The following U.S. patents utilize the displacement sensors:
U. S. Pat. No. 5,227,948 to Boon et al PA1 U.S. Pat. No. 5,247,219 to Nakagawa et al PA1 U.S. Pat. No. 5,216,308 to Meeks PA1 U.S. Pat. No. 5,142,175 to Watanabe PA1 U.S. Pat. No. 5,182,533 to Ritts PA1 U.S. Pat. No. 4,841,184 to Chen et al (the present inventor) PA1 U.S. Pat. No. 5,084,643 to Chen (the present inventor) PA1 1) Henrikson, C. H., et al, "Magnetically Suspended Momentum Wheels For Spacecraft Stabilization," AIAA paper no. 74-128. PA1 2) Robinson, A. A., "Magnetically Suspended Momentum Wheel For Satellite Stabilization," IEE Conf. Pub. 142, 2nd Conf. Adv. Mag. Mat. and Appl., September 1976, pages 131-134. PA1 3) Vischer, D. & Bleuler, H.,"A New Approach to Sensorless and Voltage Controlled AMBS based on Network Theory Concepts," Proc. 2nd Int. Symp. on Magnetic Bearings, Univ. of Tokyo, July, 1990. PA1 4) Bleuler, H., & Vischer; D.,"Magnetic Bearings with Minimum Hardware Requirement," ROMAG'91 Magnetic Bearing & Dry Gas Seals Conference, March 13-15, Washington D.C. PA1 5) Mizuno, T., et al, "Towards Practical Applications of Self-sensing Magnetic Bearings," 3rd Int. Conf. on AMBs. PA1 1) A search coil is collocated with the magnetizing coil of each electromagnet. The search coils with no current flowing in them pick up voltage signals due to flux variation in the magnetic cores. The voltage signals are insensitive to resistance change due to temperature variation. PA1 2) A velocity estimator (VE) combines two search coil signals from opposite electromagnets and the measured control current signal, and re-creates the rotor velocity in the bearing axis defined by the two electromagnets. PA1 3) A velocity trimmer cuts the high-frequency feedback gain of the velocity from VE to a desired value so to minimize rotor high-frequency excitation or noise. PA1 4) A high-frequency gain and phase compensator which may include lead-lag and/or notch filters, provides the means to tailor the stiffness and damping for controlling the lower rotor bending vibration modes. PA1 5) A feed forward control input is provided to accept synchronous rotating magnetic force commands to power amplifiers for canceling large rotor unbalance force. PA1 6) An automatic start-up scheme which involves sequential energizing of bias currents, momentarily grounding the output of the zero force seeking loop (ZFSL), and injecting a DC voltage at the input of the ZFSL, provides a means of self-starting.
The displacement sensors are expensive and prone to failure in harsh environments. Works on active magnetic bearings without using displacement sensors include the following publications:
REFERENCES:
References 1 and 2 have utilized directly a velocity sensor for controlling a thrust magnetic bearing. It was for supporting a momentum wheel where the rotor is relatively rigid or lacks high frequency vibration resonances. Since the control system was activated by rotor velocity, it was difficult to start the bearing levitation from a resting rotor.
References 3, 4 and 5 have utilized an observer algorithm which estimates the states (displacement, velocity, etc.) of the bearing system including the rotor, based on measured currents and EMFs in the magnetizing coils. The observer type of controller was difficult to tune because of many parameters involved, particularly, the coil copper resistance which vary strongly with temperature.